Generally, an automatic transmission or manual transmission mounted in a vehicle or the like is provided with a damper device for absorbing vibrations transferred from an internal combustion engine and transferring rotation to a transmission mechanism. The damper device generally includes a drive plate, a driven plate, and a coil spring present therebetween. The coil spring provided in the damper device is manufactured by winding one wire in a coil shape. The general basic shape of the cross section of the wire is a circle as a simple shape or an egg shape obtained by combining a semi-elliptical shape having, for example, a major radius a and a minor radius b with a semicircle having a radius b (see Patent Document 1).
By the way, when the wire is thicken (that is, the wire has a large cross sectional area), the durability is improved, but the wire cannot be wound so as to be accommodated in the storage space of the damper device when the wire is too thick, so it is impossible to simply thicken the wire. In addition, when the cross sectional shape of the wire is rectangular to simply increase the cross sectional area, the shearing stress or twisted stress generated by extension and contraction of the coil spring concentrates on corners and fatigue cracks are generated earlier, thereby resulting in degradation of durability.
In addition, there is proposed a coil spring having linear portions extending in an inner-outer diameter direction in the cross sectional shape of the wire (that is, a planar abutting plane is provided on the side surface of the wire) to distribute the stress generated when the coil spring contracts and adjacent wires abut against each other (see Patent Document 2).
In addition, since a coil spring provided in the damper device of a transmission or the like receives a centrifugal force during rotation, the damper device is provided with a semi-cylindrical cover for preventing the coil spring from being thrown off by the centrifugal force. However, since the coil spring is pushed against the cover during rotation by a centrifugal force, a stress is generated in the outer diameter side part of the wire by the pushing force. The contact part of the wire with the above cover receives the stress generated by the centrifugal force and the shearing stress generated by extension and contraction of the coil spring, so the durability may be affected. Accordingly, there is proposed a coil spring in which linear portions are provided in the outer diameter side part of the wire to make the shape of the outer diameter side part different from the shape of the inner diameter side part (see Patent Document 3).